COLLOID MOBILIZATION AND TRANSPORT WITHIN AN UNSATURATED SOIL CORE

The unsaturated zone is typically viewed as a buffer that minimizes groundwater contamination from chemicals applied to the ground surface. Detection of elevated contaminant concentrations in groundwater is evidence that contradicts the validity of this view. In fact, the unsaturated zone may be a source of groundwater contamination, as some research has shown that contaminants may sorb to and be mobilized by colloids. To better understand contaminant transport through the unsaturated zone, we aim to quantify effluent colloid concentrations at the base of an intact soil core during experiments in which simulated rainfall rates and soil tensions are varied. We collected a soil core (20 cm diameter, 23 cm length) from a silt clay loam in Virginia where previous studies indicated that at least 10% of the herbicide atrazine found at more than 1 m depth is associated with colloidal clay particles. In the laboratory, the bottom of the core is sealed to both a 10 µm porous plate (b.p.~20 cm) and a vacuum chamber regulated by a manometer. Influent solution (5 mM NaCl) is distributed over the core via a drip chamber. Eluent is collected at the base of the core every 15 min. using a fraction collector, and colloid concentration is analyzed using a spectrophotometer at 400 nm. Varying the tension permits different size pores to remain open. To determine which pores contribute to flow (p.v. ~2500 mL, ~2.5 days), eluted colloid concentrations are compared as -8 and -19 cm tensions are applied. Bromide breakthrough data from a 0.5-p.v. input pulse indicate that flow through macropores is significant and may impact colloid transport. To determine the effect of rainfall rate, colloid concentrations eluted from the core are compared as water is applied at 0.19 and 0.37 cm/hr. Results obtained in these initial, steady-state experiments with constant tensions and drip rates will form the basis for further exploration of the impact of transient conditions on colloid transport.